The present invention relates generally to telecommunication networks and, more particularly, to an improved high density digital network interface unit of reduced size, capable of communicating with remote transmission facilities.
Many telecommunication networks include a central office from which data, or "payload," signals may be transmitted over transmission lines to customer equipment on a customer's premises. Payload signals may comprise encoded analog or digital data.
Digital payload signals are typically sent over the transmission lines to a network interface unit ("NIU"). The NIU is located on the network side of the network interface, which is the point of demarcation between the local exchange carrier ("LEC") and the customer installation ("CI"). Where, for instance, the LEC is a local telephone company, the NIU would demarcate the point along the transmission lines where the telephone company's side of the telephone lines meets the customer's side of the telephone lines.
Electrically, the NIU is generally transparent to payload signals. However, NIU's have traditionally been used to provide special maintenance functions such as signal loopback. Signal loopback enhances carder maintenance operation by allowing the LEC, such as the local telephone company, to remotely sectionalize problems along the transmission lines.
A channel bank receives signals from the NIU and converts the payload from digital signals to analog signals. The channel bank transmits an analog signal for each channel differentially on two wire conductors known as a Tip-Ring pair.
The Bell telephone system in the United States, for example, has widely utilized a digital time-domain multiplexing pulse code modulation system known as the T-1 transmission system. In the T-1 system, the data to be transmitted over the lines, such as speech, may be sampled at a rate of 8,000 hertz, and the amplitude of each sample is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into binary form. Representative binary pulses appear on the transmission lines.
The binary form of each sample pulse consists of a combination of seven pulses, or bits. An eighth bit is added to the end of the combination, or byte, to allow for signaling.
Repetitively, each of the twenty-four channels on the T-1 system is sampled within a 125 microsecond period (equivalent to 1/8) of a second). This period is called a "frame." Since there are eight bits per channel and there are twenty-four channels, and there is one pulse at the end of each frame, the total number of "bits" needed per frame is 193. Thus, the resulting line bit rate for T-1 systems is 1.544 million bits per second.
Each frame of digital data is typically delimited by a "frame bit" (or "framing bit") or a series of frame bits. A frame bit serves as a flag, enabling line elements to distinguish the frame from the preceding frame or from noise on the line. In most framing protocols, whenever a receiving station detects the predetermined frame bit pattern, synchronization has been achieved. If the frame bit does not occur in its proper position in the data stream, frame loss has occurred and synchronization with the transmitting end has been lost.
In the T-1 protocol, a coding system is used to convert analog signals to digital signals. The coding system guarantees some desired properties of the signal, regardless of the pattern to be transmitted. The most prevalent code in the United States is bipolar coding with an all zero limitation (also called alternative mark inversion, or "AMI").
In bipolar coding, alternate one's (high bits) are transmitted as alternating positive and negative pulses, assuring a direct current balance and avoiding base-line wander. Contrasted with bipolar coding is unipolar coding, in which every occurrence of a high bit is seen as a positive pulse. In any coding scheme, a violation of predetermined coding rules generally constitutes an error.
Each T-1 transmission system carries 24 8-kB/second voice or data channels on two pairs of exchange grade cables. One pair of cables is provided for each direction of transmission. The T-1 transmission system is used in multiples "N", providing "N"-times-24 channels on "N"-times-two times-two cable pairs. The cables exist in sections, called "spans," between and beyond a series of regenerative repeaters. A channel bank at each end of a span interfaces with both directions of transmission. Incoming analog signals are thus time-division multiplexed, digitized and transmitted. When the digital signal is received at the other end of the line, the incoming digital signal is decoded into an analog signal, demultiplexed and filtered in order to reconstruct the original signal.
Payload signals are received by the telephone company and are transmitted, via the first span of transmission lines, to a series of regenerative repeaters separated by spans of transmission line. Regenerative repeaters are typically spaced every 6000 feet, connected by span lines. Each repeater receives data from the previous repeater or from the central office, but, because of transmission line losses, noise, interference and distortion, the signal will have degenerated. The repeater recognizes the presence or absence of a pulse at a particular point in time and, if appropriate, it regenerates, or "builds up," a clean new signal. The repeater then sends the regenerated signal along the next span of transmission line to the next repeater, stationed approximately one mile away.
The repeaters and span lines continue until the lines extend to the NIU. From the NIU, customer connections continue into customer premises.
In a T-1 transmission system, each span requires an NIU, and multiple spans are typically routed together. Therefore, multiple NIU's are usually placed together in the same physical location. Typically, network interface units are grouped together and mounted in a shelf, such as the Teltrend Rack-Mount Digital Shelf Assemblies Models DSA-120/A and DSA-111/A.
The telecommunications industry provides a standard for the dimensions of the above-discussed network interface units, the units often being referred to as "Type-400." According to the standard, a Type-400 NIU module is approximately 5.6 inches high, 5.9 inches long, and 1.4 inches wide. Accordingly, the telecommunications industry has also promulgated standards for the dimensions of a Type-400 mounting assembly (or shelf). According to the standard, each slot in an NIU mounting assembly is approximately 1.4 inches wide.
When additional customer interfaces or transmission lines are added to a network, it often becomes necessary to add additional network interface units in order to provide the required additional communication service. Unfortunately, the addition of Type 400 NIU modules has to date required additional shelf space. A need therefore exists for a digital network interface unit that will conserve existing shelf space while efficiently providing useful maintenance functions.